While considerable effort is continuously being expended in the improvement of dental prostheses, until now there has been no commercially satisfactory approach utilizing optical means for the taking of an impression for the purposes of fabricating such a prosthesis. The term "taking an impression" is here used to refer to the generation of an intangible model based upon which a prosthesis can be fabricated, whether or not this modelling, generally is followed by the actual fabrication of a prosthesis.
This is not to say that efforts have not been made to develop optical means for making such models or taking such impressions.
In U.S. Pat. No. 3,861,044, for example, a method has been described for taking an impression of a cavity or fitting a prosthetic device in the form of an inlay into the cavity, which comprises:
preparing the defective tooth;
producing a photographic representation of the tooth with the cavity intended to receive the inlay;
generating a signal representing the photographic image and using it to control an automatic machine tool;
filling the cavity with wax;
producing a second photographic representation of the tooth with the cavity filled with the wax;
generating another signal for the machine tool based upon the second photographic representation;
operating the machine tool to produce the inlay or insert; and
placing the machined inlay or insert in the cavity.
This approach, while demonstrating that the use of optical means has been recognized as a possibility in the odontology, has a number of drawbacks which, among others may have been the reason why it has not found commercial success.
Firstly, an ordinary photographic image is capable only of providing a two dimensional representation of the object whose shape is to be analyzed. If one wishes a three dimensional representation utilizing a photographic approach of this type, it is necessary to generate a large number of photographs, say a thousand or more, reproducing successive planes in the third dimension, the photographs representing, in turn, sections which must be accurately positioned with a precision of 100 microns or less for practical purposes.
Secondly, from the reference, it is not clear how the machine tool can actually be controlled from the photographic representations and, specifically, whether analog or digital signals are utilized, and how, for example, a numerically controlled machine tool may be operated from the photographs. Experience with the use of photographic representations has shown that they do not provide the precision, accurate positioning and resolution required for effective translation into control signals, especially for numerically controlled machine tools.
Thirdly, with the system described in that patent, the practitioner must fill the cavity with wax, a technique which may have meaning for the preparation of inlays, but which of course is meaningless when, for example, prosthetic crowns are to be prepared. Thus the method of that patent had only limited applicability, i.e. was not sufficiently versatile for widespread or even commercial use.
Finally, because the actual impression involves the introduction of a moldable material, namely wax, into the cavity, the mouth is not strictly an optical or radiation approach to the taking of the impression or the fitting. Since a second photograph is required and there is a time between the photographs, conditions may change, a factor leading to a reduction in precision. Furthermore, the difficulty of translating the two photographic representations into a three dimensional representation also creates problems with respect to precision.